CCNP ROUTE: 7. EIGRP Stub routers

Stub router: A router that should not forward traffic between two remote EIGRP-learned subnets.
             The stub router simply performs less work and reduces the Query Scope because neighbors will not send these routers any Query messages.
                     Stub routers still form neighborships, even in receive-only mode.

The "eigrp stub" command has several options. When issued simply as eigrp stub, the router uses default parameters, which are the connected and summary options.
Other options are: Static, Redistributed and Receive-only (does not advertise any routes).

The Query scope is limited using stub routers, thus reducing the EIGRP message traffic in a large network.

R4(config)#router eigrp 1
R4(config-router)#eigrp stub receive-only 
R4(config-router)#
*Mar  1 00:05:26.579: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 172.16.0.1 (Serial0/0) is down: peer info changed
*Mar  1 00:05:26.607: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 172.16.0.14 (Serial0/2) is down: peer info changed
*Mar  1 00:05:26.923: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 172.16.0.1 (Serial0/0) is up: new adjacency
*Mar  1 00:05:30.335: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 172.16.0.14 (Serial0/2) is up: new adjacency

To demonstrate that a stub router does not receive any Query messages and, if configured with receive-only, will not advertise any routes, consider the following:

R4 is configured as stub receive-only. We can verify this on R4 and R6:
R4#show ip protocols 
Routing Protocol is "eigrp 1"
...
  EIGRP stub, receive-only

R6#show ip eigrp neighbors detail 
IP-EIGRP neighbors for process 1
H   Address                 Interface       Hold Uptime   SRTT   RTO  Q  Seq
                                            (sec)         (ms)       Cnt Num
0   172.16.0.2              Se0/0             11 00:01:49   43   387  0  12
   Version 12.4/1.2, Retrans: 1, Retries: 0
   Receive-Only Peer Advertising ( No ) Routes
   Suppressing queries
2   172.16.0.10             Se0/1             11 00:07:01  162   972  0  7
   Version 12.4/1.2, Retrans: 1, Retries: 0
1   172.16.0.6              Fa0/1             12 00:07:01  551  3306  0  12
   Version 12.4/1.2, Retrans: 0, Retries: 0, Prefixes: 2
 
On R7, we can shutdown Serial0/0 so R6 (having no feasible successor to R7) would send Queries to its neighbors (R4 and R5).
But R6 knows that R4 is a stub router, so it doesn't send any Query messages out Serial0/1. R6 sends a Query only to R5 (out Fa0/1):

R6#debug eigrp packets query 
EIGRP Packets debugging is on
    (QUERY)
R6#
*Mar  1 00:16:30.475: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 172.16.0.10 (Serial0/1) is down: holding time expired
R6#
*Mar  1 00:16:45.083: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/1, changed state to down
*Mar  1 00:16:45.107: EIGRP: Enqueueing QUERY on Serial0/0 iidbQ un/rely 0/1 serno 12-12
*Mar  1 00:16:45.111: EIGRP: Enqueueing QUERY on Serial0/0 nbr 172.16.0.2 iidbQ un/rely 0/0 peerQ un/rely 0/0 serno 12-12
*Mar  1 00:16:45.119: EIGRP: Enqueueing QUERY on FastEthernet0/1 iidbQ un/rely 0/1 serno 12-12
*Mar  1 00:16:45.123: EIGRP: Enqueueing QUERY on FastEthernet0/1 nbr 172.16.0.6 iidbQ un/rely 0/0 peerQ un/rely 0/0 serno 12-12
*Mar  1 00:16:45.127: EIGRP: Sending QUERY on FastEthernet0/1
*Mar  1 00:16:45.131:   AS 1, Flags 0x0, Seq 23/0 idbQ 0/0 iidbQ un/rely 0/0 serno 12-12
*Mar  1 00:16:45.215: EIGRP: Received QUERY on FastEthernet0/1 nbr 172.16.0.6
*Mar  1 00:16:45.215:   AS 1, Flags 0x0, Seq 17/0 idbQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 0/0
*Mar  1 00:16:45.383: EIGRP: Received QUERY on Serial0/0 nbr 172.16.0.2
*Mar  1 00:16:45.383:   AS 1, Flags 0x0, Seq 19/26 idbQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 0/0
R6#

Also, regarding the routing process, R4, being a receive-only router, does not advertise the networks he knows of anymore (in this case: 192.168.44.0/24):

Before setting R4 as eigrp stub receive-only:
R6#show ip route
...
D    192.168.44.0/24 [90/2297856] via 172.16.0.2, 00:03:42, Serial0/0
     172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks
D       172.16.0.12/30 [90/2195456] via 172.16.0.6, 00:03:42, FastEthernet0/1
C       172.16.0.8/30 is directly connected, Serial0/1
C       172.16.0.4/30 is directly connected, FastEthernet0/1
C       172.16.0.0/30 is directly connected, Serial0/0
C       172.16.111.0/24 is directly connected, Loopback111
D    192.168.55.0/24 [90/409600] via 172.16.0.6, 00:03:42, FastEthernet0/1

After setting R4 as eigrp stub receive-only (route to 192.168.44.0/24 is not in R6's routing table anymore - same on R5):
R6#show ip route
...
     172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks
D       172.16.0.12/30 [90/2195456] via 172.16.0.6, 00:12:46, FastEthernet0/1
C       172.16.0.8/30 is directly connected, Serial0/1
C       172.16.0.4/30 is directly connected, FastEthernet0/1
C       172.16.0.0/30 is directly connected, Serial0/0
C       172.16.111.0/24 is directly connected, Loopback111
D    192.168.55.0/24 [90/409600] via 172.16.0.6, 00:12:46, FastEthernet0/1

Although R4 is an eigrp stub receive-only, R6 maintains the neighborship with all the routers:
R6#show ip eigrp neighbors
IP-EIGRP neighbors for process 1
H   Address                 Interface       Hold Uptime   SRTT   RTO  Q  Seq
                                            (sec)         (ms)       Cnt Num
2   172.16.0.10             Se0/1             13 00:00:10  204  1224  0  10
0   172.16.0.2              Se0/0             12 00:40:48   58   348  0  19
1   172.16.0.6              Fa0/1             10 00:46:00  274  1644  0  23